| Vegetable protein-based adhesives are a kind of environmental-friendly and formaldehyde-free adhesives.Compared with traditional aldehyde adhesives(phenol-formaldehyde,urea-formaldehyde and melamine-formaldehyde),vegetable protein-based adhesives have the advantages of environmental protection and renewable resources.However,vegetable protein-based adhesives have the problems of low gluing strength and poor water resistance,which limit its use in plywood industry.To solve these problems,in the present study,a low-price hot press peanut meal(HPM)was used as raw material to prepare a formaldehyde free peanut protein-based adhesive with high bond strength and good water resistance.The effect of different protein modified methods on the bonding strength and water resistance of peanut protein-based adhesive were also investigated.On the basis of protein modification,crosslinking agents were further added to improve the water resistance of the protein adhesive.Moreover,the effects of different preservatives on the antiseptic performance of peanut protein adhesive were also evaluated.Finally,the optimal preparation conditions for peanut protein-based adhesives were determined and applied to plywood industry.Meanwhile,the mechanism underlying the improvement of bonding strength and water resistance of the peanut protein adhesive by protein modification was preliminarily clarified.The specific conclusions are as follows:1.The optimal modified methods of peanut protein were determined.By analyzing the effect of the granularity of raw material on the bonding strength of plywood and considering the practical application environment,superfine grinding was chosen as the preparation method of raw material.Sodium dodecyl sulfate(SDS)was selected as the modifier of peanut protein-based adhesive.Single factor test and response surface methodology were used to optimize and determined the modified conditions of peanut protein-based adhesive:the modified temperature was 60℃,the modified time was 3 h,the ratio of material to liquid was 1:3,and the dosage of modifier was 3.2%.Under the preparation conditions,the bond strength of peanut protein-based adhesive prepared was improved significantly.When the peanut-based adhesive was applied to poplar plywood,the bond strength meets the requirements of classⅡplywood in the National standard GB/T 9846-2015.2.The boiling water resistance of peanut protein-based adhesive was improved significantly by using crosslinking agents.By comparing the effects of different crosslinking agents on the boiling water resistance of peanut protein-based adhesive when applied to plywood,polyamide epichlorohydrin resin(PAE)was chosen as the crosslinking agent,with an addition amount of 28%.The adhesive was applied to plywood and the boiling water resistance of plywood meets the requirements of classⅠplywood in National standard,which indicate the peanut protein-based adhesive can be applied in the outdoor environment.3.The antiseptic property of peanut protein-based adhesive was greatly improved by using the preservative.By comparing the three kinds of preservatives,boric acid zinc,copper sulfate and sodium benzoate,on the antiseptic property of peanut protein-based adhesive,boric acid zinc was found to have the best inhibitory effect on Aspergillus niger and Trichoderma viride,and 0.6%was determined as the best addition amount.In addition,the addition of zinc borate had no significant effect on the wet bonding strength of peanut protein adhesive.Plywood prepared with antiseptic property modified peanut protein-based adhesive can meet the requirements of classⅠplywood in National standard.4.The hot pressing conditions of peanut protein adhesive used in plywood were optimized.Single factor test and orthogonal design were employed to determine the effects of four pressing factors on the property of plywood:hot pressing temperature>hot pressing time>adhesive volume>hot pressing pressure,and the hot pressing temperature had significant effect on the bond strength of plywood(p<0.05).The optimal hot pressing conditions of plywood were as follows:the hot pressing temperature of 120°C,the hot pressing time of 8 min,the hot pressing pressure of 1.3 MPa and the adhesive volume of 240 g/m~2.The thermos gravimetric(TG)and differential thermal analysis(DTA)showed that the moisture content of peanut protein adhesive had evaporated completely when the temperature reached 140~150°C,namely,the adhesive was completely solidified,which confirmed the selection of hot pressing temperature.The plywood prepared by the optimized process conditions meets the requirements of classⅠplywood in National standard.Meanwhile,the optimized process technology is suitable for industrial production.5.The mechanism underlying the improvement of bonding strength and water resistance of the peanut protein-based adhesive by protein modification was preliminarily clarified.By analyzing the secondary structure and the tertiary structure of peanut protein before and after modification,the protein modification process was found that it can open the closed structures in peanut protein and exposed a lot of thiol groups and hydrophobic groups,which led to the significant increasing of the protein thiol groups content and surface hydrophobicity index.Therefore,more active groups in peanut protein adhesive can be reacted with wood by chemical and physical interactions.Finally,the bonding strength and water resistance of peanut protein adhesive in plywood were improved significantly. |
